In pulping and paper making, pulp fibers receive mechanical treatment which affects properties such as fiber length, deformation, flexibility and external fibrillation. This also affects the mechanical, structural and optical properties of sheets made from the fibers. Refining is a key step in optimizing pulps during paper making and has a number of effects on the fibers. For example refining enhances fiber-to-fiber bonding which strengthens a sheet but lowers its opacity. The refining step can also cut fibers to reduce fiber flocculation. It also deforms fibers by creating local defects in fiber walls which increases the wet web stretch but lowers the dry sheet elastic modulus.
Pulp properties such as fiber deformation and length can be changed by mechanical treatment and are distributions for a fiber population. During mechanical treatment several such property distributions change simultaneously. Instrumentation is needed which can determine on a fiber-by-fiber basis the values of more than one pulp property such as fiber length, fiber deformation and fiber flexibility. With regard to deformation, there are different measures of fiber deformation including curl and kink indices and mean and local fiber curvature. These different measures correlate to sheet properties in different ways.
Rapid measurements of fiber properties other than deformation have been made by transporting large numbers of fibers through a flow cell, past an imaging detector or photodetector. In some cells, large numbers of fibers pass through a cell at high fiber concentrations so test results are based on groups or flocs of fibers and are not fiber-by-fiber. Capillary cells are often used for fiber-by-fiber analysis. Such cells align and position each fiber but are subject to fiber clogging. Other types of cells are known for measuring fiber length and width but these cells are subject to fouling wherein resins, dirt and other chemicals present in the suspension with the fibers are deposited on the inside walls of the cell.
Sheath type flow cells wherein a liquid containing the particles to be analyzed is injected into a sheath liquid, have been used in biomedical research and also for plankton analysis but such cells are not suitable for rapidly measuring fiber length and fiber deformation in high aspect ratio fibers because such cells do not orient the fibers with a planar deformation. This does not permit a two dimensional imaging detector to reliably image a fiber, particularly one that is non-oriented.
Fiber diameter has been rapidly measured in non-imaging systems with a non-sheath type flow cell. Such cells that are thick enough to be plug resistant do not constrain fibers to a thin enough region for imaging systems to focus effectively on all fibers which results in unreliable measurements of fiber deformation and fiber length.
Although many commercial image analyzers and dedicated imaging systems are widely used for industrial inspection, some of which characterize the shape of a linear feature in a two dimensional image, none of the systems is capable of measuring the deformation or length of small deformed fibers such as pulp fibers.